1. Field of the Invention
The present invention relates to ferrous metallurgy and more particularly to furnace cooling arrangements.
The invention may be employed with particular advantage for cooling hearths, boshes, and stacks of blast furnaces.
The problem of enhancing the efficiency of cooling the blast furnace portions exposed to the most extensive heating is comparatively old and, despite numerous attempts directed to the solution thereof, has not been adequately solved up till now. This has been evidenced by grave accidents to blast furnaces which took place in a number of industrically developed countries during the last five years. Some of these accidents involved loss of human lives, and the total damage to industrial enterprises amounted to several millions of dollars. All the above accidents were due to inefficiency in cooling of a furnace shell, which in case of a lining burnout resulted in a local melting of furnace coolers and the shell and in liquid metal breaking out from the blast furnace.
2. Description of the Prior Art
To cool the lower portion of a blast furnace stack and bosh, there is still utilized a water cooling arrangement comprising box-like cooling elements disposed in vertical rows between a furnace shell and lining and its connected through a supply pipeline and a take-off pipeline to a process water main and to a cooling medium source, as disclosed in U.S. Pat. No. 3,628,509 and in FRG Pat. No. 2,041,399.
In intensified iron making processes, for instance with oxygen blast, at elevated pressures in a furnace and at a large volume thereof, the box-like coolers fail to provide effective heat removal. The box-like coolers are built in the lining, and as the lining breaks down they go out of action as well. Another disadvantage of the arrangement under consideration is that the box-like coolers ensure only local cooling. And, finally, it is impossible to provide for reliable tightness in those sections of a furnace shell where the box-like coolers are built.
In cooling a hearth and a hearth bottom of conventional blast furnaces, use is at present made of the above arrangement in combination with cooling plates disposed in an annular space between a furnace shell and a lining.
In some cases, the hearth and the hearth bottom may be cooled by watering the furnace from outside, in addition to employing cooling plates.
Provided that the lining is undamaged, the thermal loads imposed on the cooling plates are not high and the arrangement operates normally. In the event of a damaged lining, the arrangement fails to provide for sufficiently intensive removal of heat. Under these conditions, there arises a possibility of local melting of the cooling plates. When the liquid iron comes into contact with the cooling water, an explosion occurs which results in destruction of the furnace shell. As calculations have shown, the liquid iron can be prevented from breaking out of the furnace through the cooling members only if the velocity of the cooling water within the arrangement is not lower than 8 to 10 m/sec. Such being the case, the cooling of the furnace bottom portion alone requires an enormous amount of water, namely from 3000 to 4000 m.sup.3 /h at a pressure from 10 to 15 atm. It will be understood that pumps of the above capacity are provided with powerful drives and, hence, the power consumption of the arrangement is extremely high.
With these considerations in view, more promising proved to be an arrangement of evaporative cooling comprising a cooling screen formed by vertically arranged rows of plates provided with series-connected internal ducts communicating vertically and connected through supply pipelines and take-off pipelines with drum separators, as disclosed in FRG Pat. No. 1,931,957.
The internal ducts of each plate are disposed in the same plane and form natural circulation circuits with a respective drum separator. In operation, the cooling water within the internal ducts of the plates becomes heated to its boiling point and flows into the drum separator wherein the liquid phase and the steam phase are separated from each other and wherein a partial steam condensation occurs. The difference in the specific gravity of a steam-and-water mixture in the take-off pipeline and a cooled water in the supply pipeline is responsible for a natural repeated circulation. An obvious advantage of the evaporative cooling arrangement consists in a comparatively intensified circulation of a cooling agent without any pumps and additional power consumption.
It will be understood that the reliability of the evaporative cooling will be higher when more circulation circuits are provided within the plates of the cooling screen, as disclosed in U.S. Pat. No. 3,704,747. According to the specification the internal ducts of each plate are arranged in two planes, which provides for a more efficient removal of heat.
However, in the event of liquid metal reaching the plates of the cooling screen through a damaged lining, the arrangement under consideration does not provide for sufficient heat removal either, which is due to the fact that the water velocity within the natural circulation circuits should be 8 to 10 m/sec, a value which practically cannot be attained by natural circulation.
Known in the art is a blast furnace cooling arrangement which combines the advantages of the above arrangements of water cooling and evaporative cooling, as described in U.S. Pat. No. 4,061,317. This arrangement comprises a cooling screen arranged in an annular space between a furnace shell and its lining. The cooling screen is composed of plates forming vertical rows and incorporating series-connected main and additional internal ducts communicating vertically with each other. Both the main and the additional ducts have common inlets and outlets. Mounted above the cooling screen are drum separators communicating through supply pipes and take-off pipes with common inlets and outlets of the main and the additional ducts in vertical raws of the plates and forming closed circuits of natural circulation. Through distributing valves the additional ducts are connected to a process water supply main and form therewith a open circuit of forced circulation. The distributing valves are mounted at the common inlets and outlets of the additional ducts and permit a selective connection thereof to the circuits of forced circulation.
Provided the lining is in good working order, the additional ducts of the plates in each row are connected to the circuits of natural circulation, and the arrangement operates without any extra consumption of power. In the event of a damaged lining, the additional ducts are cut off from the circuit of natural circulation and connected to the open circuit of forced circulation, i.e. to the process water supply main. A high velocity of the process water within the additional ducts provides for an efficient removal of heat from the plates located in the hazardous zone, thereby preventing them from breaking down.
However, in emergency conditions the arrangement being considered suffers from the formation of scale deposits on the interior surface of the additional ducts. In long-time operation, the scale deposits grow thicker, whereby heat exchange between the plates and the furnace lining is materially affected even at high velocities of the process water. This disadvantage may be overcome by decreasing the content of salts in the water fed into the additional ducts at a rate of 3000 to 4000 m.sup.3 /h, but purification of water in such quantities is extremely expensive and will not be compensated by a longer service life of the cooling screen.